A pressure type slit screen well known in the prior art is provided with a screen cylinder 1 having elongated slit-like openings as shown in FIGS. 35 and 36. Paper material supplied by means of a pump flows in through an inlet section 2, advances to a flow passage 4 surrounding an outer circumference formed by an inside casing 3, and heavy foreign matter such as metal pieces, sand and the like in the paper material is discharged outside of the system through a trap 5 provided in a tangential direction opposite to the inlet section 2. Paper material circulating through the flow passage 4 enters into an annular screening chamber 7 formed of the screen cylinder 1 and a bearing cylinder 6 from its top portion in the direction shown by arrow 8, then it is selectively filtered while passing through the screen cylinder 1 in the process of flowing downwards, and it is discharged through an outlet section 9.
On the other hand, foreign matter such as plastics, bound fibers, wood pieces, etc. having a size unable to pass through the screen cylinder 1, would flow down by themselves through the screening chamber 7, and would be discharged through a reject outlet section 10. In addition, hydrofoil members 12 (FIGS. 42 and 43) suspended from a top of a main shaft 11 and driven by an electric motor 13 revolve continuously at a high speed along the surface of the screen cylinder to stir the paper material and remove unpassable foreign matter on the screen cylinder surface, whereby they serve to always keep the screen cylinder clean, and at the same time they disintegrate fiber flocks produced as a result of mutual aggregation of fibers by strongly stirring and promote the flow of fibers passing through the screen cylinder.
Also in FIGS. 38 and 39, to the opposite ends of the cylindrical screen cylinder 1 are mounted taper rings 15 for equipping it in a main body of a screen apparatus, by means of taper pins 16. The screen cylinder 1 is provided with circumferentially extending rows of a large number of slit openings 19 each of which extends parallel to the rotational axis of the screen cylinder. On a surface 17 of the screen facing the screening chamber 7 are provided slit openings 19 which are straight in the direction of thickness of the screen cylinder and formed by walls which are separated from each other by a preliminarily defined dimension and which are parallel to each other, and inlet corner sections 21 where the surface of the screen cylinder and the parallel walls intersect nearly at right angles.
On a rear surface 18 of the screen cylinder 1 are provided escape grooves 20 having a sufficiently large opening dimension as compared to the slit opening 19. FIG. 42 shows a structure of a screen cleaning device in the prior art, in which the hydrofoil members 12 are assembled by means of a spider 22 and a reinforcement ring 23 and they are mounted to move along a circular path across the surface of the screen cylinder.
However, in order to obtain predetermined stirring and cleaning effects based on the principle of the hydrofoil, it is necessary to extremely narrow the gap clearance between the hydrofoil member 12 and the screen cylinder to as small as 1.5-2.5 mm and to drive the hydrofoil members at a high speed of 10 m/sec. to 30 m/sec. To that end, additional support members and reinforcement members are necessitated for the purpose of assuring a rigidity for withstanding the high-speed rotation, and upon assembly also, a high degree of technique is required. Such support members and reinforcement members had various shortcomings such that since they form surface portions which cause adhesion and binding of fibers in paper material, and since they result in large power loss, counter-measures against pulsations were necessitated.
As described above, in the pressure type slit screen in the prior art, a pulsated pressure consisting of a positive pressure and a negative pressure is generated according to the principle of the hydrofoil by moving the hydrofoil members 12 along the surface of the screen cylinder 1 facing the screening chamber 7 with a narrow gap clearance held therebetween, whereby paper material is stirred and clogging of the screen plate is prevented, as shown in FIG. 37 (wherein 100 represents the direction of revolving, 101 represents clogging fiber flocks are removed, 102 represents effective flow water component, 103 represents pressure distribution and 104 represents screen inner pressure). Accordingly, the hydrofoil members 12 are revolved in order to obtain a predetermined pulsated pressure, then the paper material liquid stirred by the hydrofoils is not limited to that existing between the hydrofoils and the screen cylinder 1, but even the paper material liquid existing at a place further remote from the hydrofoils in the radial direction is stirred. Hence, power consumption for effecting necessary stirring is large.
While the power consumption is large as described above, an efficiency of power for passing paper material is lowered because the maximum stirring is effected in the neighborhood of the hydrofoils. In order to compensate for this, the hydrofoil members 12 are placed close to the surface of the screen cylinder 1. In this case, for the purpose of preventing interference between the cylinder 1 and the hydrofoil members 12 caused by torsion, these members 12 are constructed rigidly. Accordingly, if hard foreign matter should come in as mixed with the paper material liquid and should be caught between the cylinder 1 and the hydrofoil members 12, then serious damage would occur to both component members, and especially to the cylinder 1.
As described above, despite of the fact that the hydrofoil members 12 are provided in the proximity of the cylinder 1, the stirring at the surface of the cylinder 1 is insufficient, hence fibers to be passed through the cylinder 1 would flow out through the reject outlet 10, and a yield of fibers would be lowered. There was a shortcoming that the above-mentioned tendency would become more remarkable in the case of high-quality long fibers.
The hydrofoil member 12 is continuous in the direction of its rotational axis, and it passes intermittently through the proximities of the paper material outlet 9 or inlet 2. At this moment, a pressure wave shown in FIG. 37 is made to propagate through paper material pipings, and when it reaches a headbox of a paper machine, variation of a mass per unit area would occur in the direction of a flow of paper, that is, the so-called pressure pulsation problem would arise. In this case, as the power necessitated for driving the hydrofoil members 12 becomes larger, the greater becomes the energy of pressure pulsation, and therefore, the problem would become more remarkable, resulting in a shortcoming that the countermeasure for absorbing the pulsation would become more difficult.
Now, a method for making the above-described screen cylinder will be explained with reference to FIGS. 44 and 45. It is to be noted that symbols (a)-(f) in the following description correspond to symbols (a)-(f) indicated in FIG. 44:
(a) Escape grooves 20 are formed by machining them in a flat plate at a predetermined pitch.
(b) Slit openings 19 are formed by machining them nearly at the centers between the escape grooves so as to have a predetermined slit width.
(c) Edge portions and burrs at the opposite ends of the slit openings 19 are removed by means of a file or the like.
(d) A screen plate 37 is bent precisely so as to have a predetermined diameter.
(e) Joint portions in the longitudinal direction of the screen plate 37 are fixed by welding. Projected portions are removed so that a welding bead becomes flush with the surface.
(f) An escape groove 20 and a slit opening 19 are formed in the jointed portions in the longitudinal direction of the screen plate.
(g) Flanges 38 are mounted to the opposite ends of the screen plate, and the flange surface is machined into a predetermined dimension.
The method of making the screen cylinder of the prior art as described above, had the following serious shortcomings: